Re: qemu-kvm: do not allow vcpu stop with in progress PIO

[Avi Kivity <avi@redhat.com>]

On 02/09/2010 10:58 PM, Marcelo Tosatti wrote:
> You're right... this should be enough to avoid a stop with uncomplete
> PIO (and this is what happens for MMIO already). The signal will not
> be dequeued, so KVM will complete_pio and exit before entering with
> -EAGAIN. Please review and queue for stable.
>
>    

Not right enough.  This is very fragile, we depend on the kernel 
noticing the signal after completing pio but before starting execution.  
I don't think we guarantee that.

Maybe we should turn complete_pio/complete_mmio to an ioctl, so that we 
can control what happens exactly.  Or maybe it's simplest to document it 
as a feature and guarantee it.  There's some merit in it - only guest 
execution is the nonatomic part, so we only interrupt that.

> qemu upstream needs a bit more work.
>    

Could be as simple as raising a blocked exception that is unmasked by 
kvm, then entering the guest.

> -------
>
> Re-enter the kernel to complete in progress PIO. Otherwise the
> operation can be lost during migration.
>
> Signed-off-by: Marcelo Tosatti<mtosatti@redhat.com>
>
> Index: qemu-kvm/qemu-kvm.c
> ===================================================================
> --- qemu-kvm.orig/qemu-kvm.c
> +++ qemu-kvm/qemu-kvm.c
> @@ -967,6 +967,7 @@ int kvm_run(CPUState *env)
>                                   run->io.direction,
>                                   run->io.size,
>                                   run->io.count);
> +            r = 0;
>               break;
>           case KVM_EXIT_DEBUG:
>               r = handle_debug(env);
>    


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.